Best start with putting agression in screams is having a good vocalist with a pair of functioning BALLS.
Then tell him "think angry t-rex crossed with a rabid wolf"
anything less is fucking unacceptable. seriously.
and aside from that? emotion. push vocalists to give an emotional performance, ask them what their lyrics make them feel, try and do anything to bring that emotion out in their performance because that's what sells it. i'll take a shitty vocalist with a heartfelt, passionate, emotional performance over perfection. i mean look at converge, you cant make out a word jacob bannon says but it doesnt fucking matter, because you FEEL it, you KNOW he means it. no amount of saturation, distortion or processing in general can replace that.